I have a twin sister who also likes to cosplay. I want ideas of a good set of characters the two of us can cosplay. I thought it would be cool if we did matching cosplays that work on the idea that we are twins.

-Rin and Len Kagamine from Vocaloid.
-Like Mai said, The twins from Ouran High School Host Club
-Whatever Naruto characters can use the Clone thing (Not a fan so I have no idea what it's called), cosplaying the same one but one of you be the clone.
-Kyoko Sakura from Madoka Magica cloning (She also can use cloning like the Naruto thing)

You could also do two different outfits of the same character, like perhaps a transformed/untransformed magical girl (good for mirror shots). That covers everything from Sailor Moon to Cutey Honey (http://www.internationalhero.co.uk/c/cuteyhoney1.jpg <-- same girl!). Another cute idea could be the same character in alternate colors/outfits from a fighting game like Street Fighter or King of Fighters.